Please scan through the reading on variable arity procedures.
Here is the
display-line procedure from the reading.
;;; Procedure: ;;; display-line ;;; Parameters: ;;; 0 or more values ;;; Purpose: ;;; Displays the strings terminated by a carriage return. ;;; Produces: ;;; Nothing ;;; Preconditions: ;;; (none) ;;; Postconditions: ;;; The standards (define display-line (lambda arguments (let kernel ((rest arguments)) (if (null? rest) (newline) (begin (display (car rest)) (kernel (cdr rest)))))))
a. Try out some other calls to
display-line to check what it
prints. For example, try the following:
(display-line "going" "going" "gone") (display-line "countdown:" 5 4 3 2 1 "done") (display-line) ;; apply display-line to no arguments
b. Explain your results.
The current version of
display-line prints all text together
without spaces. Modify the code, so that one space is printed between any
two adjacent values supplied as arguments to
For instance, after your modifications, the example from the reading will
change. It will now be ...
> (display-line "+--" "Here is a string!" "--+") +-- Here is a string! --+
You may not use
display-separated-line in your answer to
Define and test a procedure named
call-arity that takes any
number of arguments and returns the number of arguments it received
(ignoring their values):
> (call-arity 'a #\b "c" '(d)) 4 > (call-arity 0.0) 1 > (call-arity) 0
a. What happens if you invoke
giving it any arguments?
b. What happens when you give it only one argument?
c. What happens when you give it two arguments?
c. What happens when you give it three arguments?
Define and test a procedure
clicker that takes one or more
arguments, of which the first must be an integer and each of the others must
be either the symbol
'up or the symbol
Clicker should start from the given integer, add 1 for
'up argument, subtract 1 for each
argument, and return the result:
> (clicker 17 'up 'up) 19 > (clicker -12 'down 'up 'down 'down 'down) -15 > (clicker 100) 100
In writing, we often separate the last element of a list using a
different separator than for the prior elements. For example, we
might separate the all but the last element with commas and the last
element with "and". Extend
that it requires two parameters (the default separator and the final
separator) and supports as many the user clinet to provide.
Tuesday, 31 October 2000
Disclaimer Often, these pages were created "on the fly" with little, if any, proofreading. Any or all of the information on the pages may be incorrect. Please contact me if you notice errors.
This page may be found at http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2000F/Labs/variable-artiy.html
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